1. Field of the Invention
This invention relates, in general, to protective devices for electrical apparatus and, more specifically, to sudden pressure relays for power transformers.
2. Description of the Prior Art
Sudden pressure relays are used in connection with electrical apparatus, such as power transformers, to detect when a sudden increase in gas pressure within the electrical apparatus enclosure has occurred. Normally, the gas space is located above the level of a fluid dielectric within the enclosure. These sudden increases in pressure are usually caused by an internal fault within the apparatus. Thus, the sudden pressure relay is generally connected to control and alarm circuits which are associated with the high-voltage circuits entering the electrical apparatus.
One conventional sudden pressure relay includes a pressure sensing bellows which is located in substantially unrestricted flow communication with the gas. The bellows is surrounded by a gas chamber which is connected to the gas in the apparatus enclosure through a pressure equalizing orifice. The orifice throttles rapid increases in gas pressure and keeps the pressure of the gas around the outside of the bellows below that of the gas on the inside of the bellows, thus allowing the bellows to expand. When the rate of gas pressure increase is slow, the gas pressure around the outside of the bellows increases as fast as the pressure on the inside of the bellows, therefore the bellows will not expand on slowly rising gas pressures.
Sudden pressure relays of the type described cannot differentiate between a sudden increase in gas pressure as a result of internal faults and a sudden increase in gas pressure caused by excessive splashes and waves occurring in the fluid level due to large physical disturbances originating outside the apparatus enclosure. Such disturbances are known to occur during seismic or earthquake conditions. However, since it is not usually desirable for the sudden pressure relay to erroneously indicate an internal fault under seismic conditions, conventional sudden pressure relays may not be desirable for use where seismic disturbances are expected. Therefore, it is desirable, and it is an object of this invention, to provide sudden pressure relays which will not be activated by seismic disturbances.